The present invention relates to a feed control apparatus of a fabric feed device which drives directly a feed dog by a pulse motor controlled by open loop control system.
Conventionally, fabric feed devices for sewing machines obtain horizontal feeding motion of the feed dog by converting rotation of a main motor with a large torque into swinging motion. A converting mechanism for providing this swinging motion is constituted of a large number of parts such as a cam and a forked link, therefore a defect can result when a stitch pattern gets out of shape due to accumulated errors in the feed motion to be transmitted. Also, the conventional feed devices require a large space for installation in the bed of the sewing machine, resulting in a hindrance in making the sewing machine lighter and smaller.
Then, to eliminate the above-mentioned defect, an apparatus which drives a feed dog using a pulse motor as an independent driving source of the main motor is proposed and disclosed in the Japanese Patent Publication (examined) No. 57-30026. This apparatus detects each phase of rotation of an upper shaft and directly drives a pulse motor by an open loop control system in synchronism with the phase detection signal, thereby freely controlling feed motion of work fabric.
Control of the pulse motor by the open loop control system is performed by commanding the rotary angle of the pulse motor with reference to the number of steps or unit rotating motion. Accordingly, in operating the sewing machine, the pulse motor is set to an original or reference position once, and thereafter the number of command pulses is controlled assuming that the pulse motor is rotating in accurate response to the command pulses.
However, in a feed device employing a small-sized pulse motor with a small torque, an overload is applied to the feed dog due to forced feeding when starting the sewing machine or due to transfer of a thick part of work fabric, and thereby the pulse motor sometimes falls into step-out so as to be out of sync with respect to the reference position (loss of synchronism). If the pulse motor falls out of synchronism with respect to an initial reference position once, despite the fact that the relation between the rotary position of the pulse motor and the number of commanded steps is out of order, a controller gives command pulses assuming that the pulse motor is in synchronous state, and the feed dog is permitted to move within a predetermined range of horizontal feed motion and, therefore after the feed dog has reached the limit position and stopped, only the excited state of the pulse motor is changed and the pulse motor remains out of synchronism.
For this reason, once the pulse motor, which is out of sync due to overloading remains out of sync during continued operation. even after removal of the overload, and cannot give an accurate amount of horizontal feeding motion to the feed dog, therefore there remains a defect that the stitch pattern gets out of shape.
To prevent the above-described step-out non-synchronization of the pulse motor with respect to a reference position, a method is proposed wherein the overload to be applied to the feed dog is applied to the pulse motor through an escape spring without applying it directly to the pulse motor. However, in such a method, the pulse motor is required to rotate against a maximum tension of the spring without stepping-out. For this purpose, the maximum tension of the spring is required to be smaller than the step-out torque of the pulse motor. However, reduction in the spring constant means that the feed dog does not synchronize easily with the rotation of the pulse motor by a small external force, resulting in deterioration of the stitch pattern, and also means that the torque of the pulse motor is not used fully as a feeding force. Also, an increase in the step-out torque makes the pulse motor larger in size, raising a problem in weight and mounting.
In order to eliminate this defect, as described in U.S. Pat. No. 4,696,247, one of the inventors of the present invention proposed a feed device including a stopper disposed at a stoppage position corresponding to at least one of an upstream limit position and a downstream limit position of the horizontal feed motion of a feed dog. A feed control, having a excitation control, switches the excitation mode of a stepping motor which draws the feed dog to a specific excitation mode at least at the start and the end of the horizontal feed motion.
In the above-noted feed device, the stepping motor can be resynchronized to a reference position at the start-up of the sewing machine and during sewing operation. However, in the case where the asynchronous state of the stepping motor is induced by forceably feeding the fabric at the start-up of the sewing machine, feeding motion is carried out in the asynchronous state until the feed dot arrives at the downstream stoppage position. Therefore, the feed device cannot feed the fabric accurately.
Also in the case where the asynchronous state is induced by the feeding resistance due to a thick part of the fabric, feed motion is carried out in the asynchronous state until the feed dog arrives at the downstream stoppage point.
In order to solve the above-noted problems, at the start-up of the sewing machine, and after correcting an asynchronous state of the stepping motor, feeding motion should be carried out. Similarly, during sewing operation, after correcting an asynchronous state of the stepping motor at both the start or the end of the feed motion, continued feeding motion should be carried out. However, this patient publication does not disclose such a technical concept.